Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 31, 2000
Publication Date: N/A
Interpretive Summary: Use of low levels of antibiotics as a dietary supplement is common in the cattle industry. Beneficial effects of feeding antibiotics include increases in efficiency of gain and alterations in composition of gain. These positive responses to antibiotics are most obvious when negative environmental influences are present, such as transportion and changing the feed being offered. Research has indicated that the antibiotics act by changing the total amount of bacteria in the digestive tract and possibly, decreasing the size of the intestines. Little research into the effects of feeding antibiotics on the intestinal tissues in steers has been done. This report shows that the generally accepted concept that intestinal mass is decreased in size in response to dietary feeding of subtherapeutic concentrations of CTC due to decreased proliferation rate of the small intestinal tissues may not be accurate. However, these findings are in agreement with the concept that the beneficial effects of subtherapeutic feeding of antibiotics are due to decreases in the mass of the highly metabolically active tissues that make up the intestinal tract.
Thirty-two beef steers were used to determine the effects of chlortetracycline (CTC) and dietary crude protein CP on visceral organs. Steers received dietary treatments consisting of either 10 or 13% CP diets top-dressed with a corn meal carrier containing either 0 or 350 mg CTC. After 84 d, steers were slaughtered, visceral organs removed, and separated, wet weight was recorded, total RNA, total DNA, tissue dry matter, and tissue N content were determined, and tissue sections were prepared for immunohistochemical analysis. Rumen and abomasum weights and small intestinal (SI) length were greater in steers fed the 13% CP diet than those fed the 10% CP diet on both an absolute weight basis as well as a percentage of empty body weight (EBW). Dietary administration of CTC decreased SI weight both on an absolute and corrected for EBW basis. Nitrogen and RNA concentrations of the small intestinal segments were unaffected by dietary administration of CTC, however because of increases in the tissue DNA content, the ratio of N to DNA was decreased in the small intestinal segments of the CTC treated animals. The observed decrease in small intestinal epithelial mass does not appear to be due to alterations in cell proliferation rate but rather cell size. Consistent with this finding, cell proliferation, as determined by Ki67 antigen staining, was not affected by dietary treatment. Chlortetracycline administration results in decreased small intestinal mass that may be a result of decreased cell size.